1. Field of the Invention
This invention relates to a process for forming particulate materials, especially fiber materials, and binders into articles that are recyclable after comminution.
2. Background Art
Fiberboard is often manufactured using pulverulent binders based on crosslinkable and noncrosslinkable addition polymers. Noncrosslinkable binders used include for example polypropylene (PP) or polyethylene (PE). Since polyethylene and polypropylene are thermoplastic, fiberboard manufactured using polypropylene or polyethylene deforms at high temperatures because of its low heat resistance.
In contrast, fiberboard manufactured using crosslinkable binders such as phenolic resins or styrene-acrylate copolymers having crosslinkable groups possesses very good thermal deformation resistance (EP-A 894888, DE-A 10014399). Fiberboard of this type has the disadvantage, however, of being impossible to recycle owing to the irreversible, chemical crosslinking. Fiberboard of this type can at most be comminuted and ground and the material obtainable thereby reused as a filler.
EP-A 807704 discloses a process for recovering fibers from fiber materials wherein the fibers are ionically bonded by a carboxyl-functional polymer whose carboxyl groups are crosslinked by an alkaline earth metal ion. The process comprises treating the fiber material with an aqueous solution of a salt whose anion forms a sparingly soluble salt with the alkaline earth metal cations and then removing the debindered fibers.
DE-A 19535792 describes a process for preparing recyclable fiber composites by bonding the fibers together using an aqueous polymer dispersion whose film is converted into an aqueous addition polymer solution upon a change of pH. After the bonding agent, or binder, has been converted into an aqueous solution, the fiber material can be removed and fibers and binders reused.
It is an object of the present invention to provide a process for forming articles, especially from fiber materials, which possess a high thermal deformation resistance and are recyclable.
These and other objects are achieved by a process for forming particulate materials and binders into articles that are recyclable after comminution, wherein the particulate materials are treated with a binder composition containing
A) one or more polymers consisting at least one comonomer unit selected from vinyl esters of branched or unbranched alkyl carboxylic acids of 1 to 18 carbon atoms, acrylic or methacrylic esters of branched or unbranched alcohols of 1 to 15 carbon atoms, dienes, olefins, vinyl aromatics and vinyl halides, and further containing 4 to 60% by weight, based on the total weight of the copolymer, of one or more units containing polar groups, and
B) 4 to 25% by weight, based on the total weight of polymer and filler, of one or more fillers having a polar surface capable of interaction with the polar groups of polymer(s) A). The binder-containing particulate materials are then formed into articles and consolidated.
Useful vinyl esters include vinyl esters of branched or unbranched carboxylic acids of 1 to 18 carbon atoms. Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of xcex1-branched monocarboxylic acids of 5 to 11 carbon atoms, for example VeoVa9R or VeoVa10R vinyl esters (trade names of Shell). Vinyl acetate is particularly preferred.
Useful monomers from the group of the esters of acrylic acid or methacrylic acid include esters of branched or unbranched alcohols of 1 to 15 carbon atoms. Preferred methacrylic esters or acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate, norbornyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate and norbornyl acrylate.
Useful dienes include 1,3-butadiene and isoprene. Examples of copolymerizable olefins are ethene and propene. Useful vinyl aromatics for copolymerization include styrene and vinyltoluene. Vinyl chloride is the customary vinyl halide.
Useful polar groups are, for example, carboxyl, hydroxyl, phosphonate, sulfonate or NH groups.
Useful carboxyl-functional comonomers for copolymer A) include ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid and maleic acid. The carboxyl function may also be introduced by copolymerizing maleic anhydride into the copolymer A). Useful hydroxyl-functional comonomers include hydroxyalkyl acrylates and hydroxyalkyl methacrylates having a C1- to C8-alkyl radical, preferably hydroxyethyl acrylate and methacrylate, hydroxypropyl acrylate and methacrylate, and hydroxybutyl acrylate and methacrylate. Useful NH-functional comonomers include (meth)acrylamide, diacetoneacrylamide, maleimide, alkyl maleamate, alkyl fumaramate, maleamide, fumaramide, vinyl glutaramate, vinyl succinamate, allyl glutaramate, allyl succinamate, dimethylaminoethyl methacrylate, dimethylaminopropylmethacrylamide, trimethylammonium methyl methacrylate chloride, and trimethylammonium propylmethacrylamide chloride. A useful phosphonate-functional comonomer is vinyl phosphonate. Useful sulfonate-functional comonomers include vinyl sulfonate, styrenesulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid.
Preference is given to carboxyl-functional comonomer units, preferably those derived from acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid and maleic anhydride. The fraction of functional units in copolymer A) is in the range from 4 to 60% by weight and preferably in the range from 5 to 25% by weight, each percentage being based on the total weight of the copolymer. Each polar comonomer may be used to the exclusion of one or more of other polar comonomers.
Preference is given to the polymer compositions specified below for copolymer A), which additionally includes, in the foregoing amounts, the functional group-containing monomer units previously described:
vinyl acetate polymers; vinyl ester-ethylene copolymers, such as vinyl acetate-ethylene copolymers; vinyl ester-ethylene-vinyl chloride copolymers where the vinyl ester component is preferably vinyl acetate and/or vinyl propionate and/or one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, vinyl esters of an alpha-branched carboxylic acid of 5 to 11 carbon atoms, especially vinyl versatate (VeoVa9R, VeoVa10R); vinyl acetate copolymers with one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, vinyl esters of an alpha-branched carboxylic acid of 5 to 11 carbon atoms, especially vinyl versatate (VeoVa9R, VeoVa10R), which optionally contain ethylene as well; vinyl ester-acrylic ester copolymers, especially with vinyl acetate, butyl acrylate and/or 2-ethylhexyl acrylate, which optionally contain ethylene as well; vinyl ester-acrylic ester copolymers with vinyl acetate and/or vinyl laurate and/or vinyl versatate and acrylic esters, especially butyl acrylate or 2-ethylhexyl acrylate, which also optionally contain ethylene as well.
Particular preference is given to (meth)acrylic ester and styrene polymers: copolymers with n-butyl acrylate and/or 2-ethylhexyl acrylate; copolymers of methyl methacrylate with butyl acrylate and/or 2-ethylhexyl acrylate and/or 1 ,3-butadiene; styrene-1,3-butadiene copolymers and styrene-(meth)acrylic ester copolymers such as styrene-butyl acrylate, styrene-methyl methacrylate-butyl acrylate or styrene-2-ethylhexyl acrylate, where the butyl acrylate used can be n-, iso- or tert-butyl acrylate.
Most preference is given to compositions having a carboxyl-functional styrene-n-butyl acrylate and/or a carboxyl-functional styrene-methyl methacrylate-n-butyl acrylate copolymer as copolymer A).
The comonomers and their weight fractions are chosen so that in general the resultant glass transition temperature Tg is in the range from 40xc2x0 C. to 110xc2x0 C. in the case of polymers used in powder form and in the range from xe2x88x9230xc2x0 C. to 95xc2x0 C. in the case of polymers used in aqueous dispersion or solution. The glass transition temperature Tg of the addition polymers can be determined in a known manner by differential scanning calorimetry (DSC). The Tg can also be approximately predicted by means of the Fox equation, i.e. aAccording to T. G. Fox, Bull. Am. Physics Soc. 1, 3, page 123 (1956):
1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn,
where xn is the mass fraction (% by weight/100) of monomer n and Tgn is the glass transition temperature in kelvin of the homopolymer of monomer n. Tg values of homopolymers are given in the Polymer Handbook 2nd Edition, J. Wiley and Sons, New York (1975).
The addition polymers may be prepared by known free-radically initiated polymerization processes, for example by solution polymerization, aqueous suspension polymerization, or aqueous emulsion polymerization. Preference is given to emulsion polymerization. The solutions or dispersions may be dried using all common drying processes: spray drying, drum drying, freeze drying, belt drying, coagulation with subsequent fluidized bed drying. Preference is given to using spray drying and drum drying processes.
The addition polymers may be used in dry pulverulent form, in the form of an aqueous dispersion, or in water-dissolved form. The addition polymers are preferably used as powders.
Useful fillers include pulverulent organic and inorganic materials having a polar surface, generally having a hydroxyl-functional surface. Examples of inorganic fillers are titanium dioxide, aluminum oxide, aluminum hydroxide, magnesium oxide, quartz flour, glass fiber, glass powder, chalk, talc, lime, calcium carbonate, magnesium carbonate, barium sulfate, kaolin, dolomite, magnesium silicate, calcium silicate and silicates such as aluminum silicate. Examples of organic fillers are wood flour, cellulose, starch and also synthetic polar polymers having a Tg greater than 90xc2x0 C., for example polyesters, crosslinked carboxyl- or sulfonate-functional styroacrylates, the latter especially as hollow spheres, plasma-treated polymer powders and also polymer resin wastes. The average particle size of the pulverulent fillers is generally in the range from 5 nm to 900 xcexcm.
The binder composition contains 4 to 25% by weight and preferably 5 to 20% by weight of filler, each percentage being based on the total weight of addition polymer and filler. The binder composition may be prepared by mixing the aqueous dispersions or solution of the addition polymer with the filler. In the process, the filler is often applied in the form of a paste, in a blend with surfactants. Pulverulent binder compositions may be prepared by drying these aqueous mixtures. It is also possible for polymer powder and filler powder to be mixed or conjointly extruded from an extruder; preferably in such a way that the mixing temperature is lower than the minimum filming temperature MFT of the addition polymer in order that complete wetting of the filler particles with the addition polymer may be avoided.
The binder composition is useful for forming articles from particulate materials such as fiber materials or particulate materials selected from mineral materials, plastic materials or natural materials such as wood shavings, cork particles, glass particles or glass powder, especially recycled glass and glass microballoons, or from combinations thereof. The preferred use is as a binder for fiber materials. Useful fiber materials include natural or synthetic raw materials. Examples thereof are manufactured fibers based on fiber-forming polymers such as viscose, polyester, for example chopped polyester fibers, polyamide, polypropylene, and polyethylene fibers. It is also possible to use mineral fibers such as glass fibers, ceramic fibers, and/or carbon fibers. Examples of natural fiber materials are wood, cellulose, wool, cotton, jute, flax, hemp, coir, ramie and sisal fibers. The fibers can also be used in the form of woven textiles, in the form of yarns, or in the form of nonwovens such as nonwoven scrims or formed-loop knits. These nonwovens may, if appropriate, be mechanically preconsolidated, for example needled.
Depending on their intended application, the articles are formed at room temperature or at elevated temperature, if appropriate, under elevated pressure. The temperature for consolidating the formed articles is generally in the range from 20xc2x0 C. to 220xc2x0 C. When an elevated consolidation temperature is used, it is preferably in the range from 90 to 220xc2x0 C. When the formed articles are produced under pressure, it is preferable to employ pressures in the range from 1 to 200 bar. The binder composition is generally used for forming the articles in an amount of 5 to 50% by weight, based on the material to be bound. The binder quantity depends on the substrate to be bound and is preferably between 10 and 40% by weight in the case of polyester and cotton fibers, and preferably in the range from 20 to 40% by weight in the case of natural fibers such as hemp, flax, sisal, jute, for example for automotive interior applications. In the case of glass and mineral fibers and also with other mineral materials, such as glass balls, the preferred range is between 10 and 30% by weight. A further application is the manufacture of HDF and MDF and of wood extrudates, for which the binder composition is mixed with wood particles and subsequently extruded.
Fibrous articles are formed by mixing a pulverulent binder composition with the fibers, spreading out the fiber-powder mixture by customary methods of nonwovens technology, if appropriate after carding of the fiber-powder mixture and needling, and bonding at elevated temperature, with or without employment of pressure and/or superheated steam. The fiber bonding can also be effected by sprinkling the pulverulent binder composition into a woven fabric, a nonwoven scrim or into a previously laid-down fiber bed. After carding of the fiber-powder mixture and, if appropriate, needling, the bonding powder may be melted and cured at elevated temperature, optionally with additional employment of pressure and/or superheated steam.
When an aqueous binder composition is used, it is applied in a known manner to the particulate material or to the nonwoven by spraying, doctor coating, padding or dipping and the particulate material or nonwoven is then bonded at elevated temperature with or without employment of pressure.
It is surprising that the combination of fillers having a polar surface and polymers having polar groups provides a high thermal deformation resistance at temperatures above 70xc2x0 C. without the water imbibition of the formed articles increasing significantly. Formed articles produced by this process can be comminuted for recycling and reused for forming articles, since the high thermal deformation resistance here is due to a physical interaction. Formed articles bonded in this way, in contradistinction to chemical crosslinks, remain processible at high temperatures upon reuse.
To recycle the formed articles, they are comminuted, preferably granulated or pulverized. The materials obtained thereby can again be processed into formed articles. Alternatively, the materials obtained thereby may be used as binders for forming articles from particulate materials. Recycling preferably involves the addition of further binders or of the just described binder composition of A) and B) or of virgin fiber, the latter sometimes necessary since comminution may cause the fiber form to change.
The examples hereinbelow illustrate the invention: